Astronomy Benalla
Reports - 2011
Astronomy Benalla  General Meeting Presentations - Wednesday 15th June 2011 Two presentations were given by President Rupe Cheetham Presentation1: Constellation of the month MUSCA   The Fly Originally called  Apis  the Bee  in Johann Bayer’s catalogue in 1603, was later renamed Musca Australis (Southern Fly). By 1950 the International Astronomical Union (IAU)  knew it as simply Musca. This constellation is found immediately south of the easily located Crux (Southern Cross). Musca is a relatively small constellation ranking 77th out of 88. It has 6 main stars in its asterism that remotely looks like a winged insect: The brightest star alpha Muscae, magnitude 2.89, is a Beta Cephei variable with a heart beat every 2.2 hours. Beta, eta, and theta Muscae are all binary stars. Most major stars are 300 to 400 light years distant with lambda, the tail of the fly and a white sub-giant, only 130 light years away. Theta, a Wolf-Rayet star is found at about 6200 light years. Musca has several globular clusters that can be seen with binoculars - NGC4833, NGC4372  and Caldwell 105.  These are at distances of 18,000 to 26,000 light years. Small aperture telescopes may pick up 2 open clusters, NGC4185 and NGC4463. Larger scopes are needed to view the planetary nebulae NGC5189 (aka the Spiral Planetary Nebula), IC4191  and MyCn19 (the Hour Glass Nebula).   One large exoplanet 6.8 times larger than Jupiter has been located orbiting star HD111232 and completes an orbit in 1143 days. This exoplanet was discovered in 2004 by the Radial Velocity method. NGC4373 (aka the Dark Doodad) is a dark, dense molecular gas and dust cloud adjacent to gamma Muscae. Presentation 2:    “Number 7”         Uranus - The Odd Planet Uranus as seen by the Voyager 2 space craft, is the blue planet. This colour is caused by methane crystals in its upper atmosphere absorbing the red light spectrum. Uranus was discovered by William Herschel on 13th March 1781 and was the first planet to be discovered using a telescope. Herschel realized it was a planet when, using higher power optics, it’s size grew; this doesn’t happen with stars.  Initially he named it Georgium Sidis  but by 1850 Uranus (father of Saturn) was accepted, this being the Latinized version of Greek for God-of-the-Sky (Ouranus). Uranus is at an average distance from the Sun of 2.88 billion kilometers and has a diameter of about 51,000 kilometers. Astronomers estimate it consists of 98% hydrogen and helium, with minor amounts of ammonia and water. The make up of a small core is still in doubt - probably liquid rock. Uranus is the coldest of the solar planets, giving off less heat than it absorbs from the Sun than that of any other.  Temperature at the cloud top is about 49˚ K or –224˚ C. Uranus is seen as odd because its axis of rotation is angled at 98 degrees to it’s orbital plane (retrograde rotation) so that the poles have an alternate summer & winter of 42 Earth years. For it’s size it rotates very fast having a day of 17 Earth hours. Uranus has 13 very faint rings and 27 moons. The largest moon Triton is approx ½ the diameter of our Moon. Uranus’s magnetic field is inclined to its rotational axis by 59 degrees and is offset from its centre by about 1/3rd of its diameter. 
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